Steady rests of this kind have been disclosed, for example, in EP 2848359 A1, by means of which a rotationally symmetrical workpiece is centred in the space and supported. Workpieces of this kind are shafts or hollow bodies, the outer jacket surface of which is to be machined using tools thereby generating considerable machining forces that act on the clamped workpiece and cause it to undergo flexure. Often, workpieces of this kind are several meters in length, as a result of which the steady rests required for supporting the workpieces are arranged at a specified spacing from one another in the area of the machine tool bed of a machine tool.
The machine tool bed of a machine tool only provides a limited space, however, which needs to accommodate additional elements as a result of which the size of the steady rest must be adapted to the available space in the machine tool. Furthermore, the steady rests must be aligned offset in a vertical direction or to the side by a slight amount in relation to the machine tool in order to encompass and centre the workpiece to be held from underneath.
The steady rests designed for this purpose chiefly consist of two housing shells in which three steady rest arms are arranged and can be moved. The steady rest arms are in a driving, active connection with a pneumatically or hydraulically operated pressure piston, by means of which the three steady rest arms can be moved synchronously using a guide slide in the direction of the workpiece to be clamped or away from it.
The middle steady rest arm is moved axially to and fro accordingly and the two outer steady rest arms make contact with the outside of the guide slide and perform a swivelling movement when the guide slide is actuated. This is because a control track is worked onto two opposite outside parts of the guide slide, and one of the free ends of each outer steady rest arm makes contact with this by means of arranged rollers, and thereby roll along the control track. The advance movement of the guide slide consequently causes the two outer steady rest arms, which are mounted on the housing shells in a rotating arrangement, to be moved towards or swivelled away from the workpiece.
As soon as the three steady rest arms encounter the surface of the workpiece simultaneously, clamping forces are applied by means of which the workpiece is held. These clamping forces as well as the movements of the three steady rest arms are achieved by means of the single-piece design of the actuation piston. As a result of the single-piece configuration of the actuation piston, it projects from the plane formed by the housing shells in order to achieve the optimum or maximum advance travel or stroke distance, with the effect that the axial advance travel or swivelling range of the three steady rest arms is as large as possible. The larger the diameter of a workpiece to be clamped, the longer the distance that the actuation piston has to cover.
However, only a limited amount of space is available in a machine tool and this is predominantly determined by the machine tool bed and the profile of the workpiece to be clamped, which means that the steady rests provided can only clamp a particular size of workpiece.